Millions of persons are afflicted with chronic forms of arthritis which are thought to involve auto-immunity to constituents of the joints or connecting tissues of the body. These conditions include rheumatoid arthritis, ankylosing spondylitis, Reiter's syndrome and other forms of reactive arthritis. The etiology of these diseases is not known, but previous infection with various microbes seems to act as an inciting circumstance in genetically susceptible individuals. For example, patients with rheumatoid arthritis may show unusual reactivity to mycobacterial antigens, and immunisation with the BCG strain of mycobacteria was found to lead to arthritis in 15 of 150 individuals. Ankylosing spondylitis has been associated with infection by Klebsiella or Yersinia species of bacteria and other cases of arthritis by Salmonella, Shigella, etc. There is no evidence of active infection of joints by these microbes in the vast majority of cases and it has been postulated that microbial infection may trigger an apparent auto-immune response of the individual against his own antigens present in the joints. Adjuvant arthritis (AA) is an experimental model of arthritis inducible by immunizing susceptible strains of rats to Mycobacteria. The disease which develops about twelve days after immunisation has many of the features of rheumatoid arthritis and AA has been considered to be a model of rheumatoid arthritis.